BIND 9.18.21 (and later) fails to exit on NetBSD when using unconfigured address

Summary

While running BIND 9.18.x on NetBSD, I noticed that when either transfer-source-v6 and / or notify-source-v6 were given an address that wasn't configured on the system, BIND would fail to exit unless given SIGKILL.

BIND version affected

This problem was noticed with versions of BIND earlier than 9.18.21, but 9.18.21 was what I was running when I discovered the reason why BIND would not exit.

Steps to reproduce

1. Set notify-source-v6 to an IPv6 address that isn't configured on the system.
2. Try /etc/rc.d/named9 restart, or rndc stop
3. Observe that BIND doesn't exit.

What is the current bug behavior?

What is the expected correct behavior?

Relevant configuration files

Relevant logs